Porous membrane and recording medium, as well as process for preparing same

ABSTRACT

The present invention relates to a process for preparing a microporous membrane comprising the step of impregnating said microporous membrane with a solution, wherein said solution comprises cationic compounds for modifying the chemical and/or physical properties of said microporous membrane. The invention further relates to image recording materials, in which these microporous membranes are used.

RELATED APPLICATIONS

This application claims priority from International application numberPCT/NL2006/000405, filed 4 Aug. 2006; which claims priority fromEuropean application number EP 05076831.6, filed 5 Aug. 2006; each ofwhich is hereby incorporated herein by reference in its entirety for allpurposes.

FIELD

The present invention relates to a process for preparing a porousmembranes by curing compounds through radiation. The invention furtherrelates to image recording materials in which these porous membranesthus produced are used, in particular as an ink-receiving layer. Theinvention also relates to the use of said membranes and said recordingmedia.

BACKGROUND

Several examples can be found in which curable mixtures are used toproduce inkjet recording media. EP-A-1 289 767, EP-A-1 418 058 andEP-A-1 477 318 disclose layers, cured by UV or other radiation in whichthe porous character is provided by organic or inorganic particles inorder to obtain sufficient solvent uptake. However, application ofinorganic particles can cause physical weakness of the layer, resultingin cracking or breaking of the layer.

EP-A-0 738 608 describes curing compositions containing a water solublehigh-molecular weight compound, but these compositions yield solidlayers and therefore do not dry quickly.

WO-A-99/21723 discloses a substrate coated with a binder dissolved in anaqueous solvent mixture, which layer is then cured, and teaches that anyamount of solvent is suitable, and there is no limit to the degree inwhich the binder is diluted before curing.

WO-A-01/91999 and GB-A-2 182 046 disclose a curable inkjet coating thatis cured after drying the coating.

U.S. Pat. No. 6,210,808 describes an inkjet recording sheet wherein acolloidal suspension of water-insoluble particles and water-insolublemonomers/prepolymers is cured.

U.S. Pat. No. 6,734,514 discloses a radiation curable coating for inkjet printing comprising water insoluble latexes.

Another method is the application of foamed layers as in for exampleEP-A-0 888 903.

In the above mentioned prior art the receiving layer is not isolated,but formed directly on the substrate after drying and/or furthermanipulation of the coated substrate. It would be advantageous, to beable to isolate such a layer as a membrane which can be used as such orwhich can be applied to a substrate in a separate process to giveadvantageous properties, for instance as an inkjet receiving medium.Membranes are produced by various methods such as dry and wet phaseinversion of polymeric solutions, stretching of homogeneous, partiallycrystalline polymer films, sintering of particulate materials, thermalgelation of homogeneous polymer solutions and by radical polymerizationwith simultaneous phase separation by irradiation or thermal initiation.Of these methods the wet phase inversion method—in which a polymersolution is contacted with a precipitating agent or non-solvent causingseparation into a solid polymer-rich phase and a liquid solvent-richphase—is by far the most widely used technique for obtaining porousstructures. Examples in which this technique is applied can be found inWO-A-98/32541, WO-A-2005/016655, U.S. Pat. No. 4,707,265, U.S. Pat. No.6,079,272, EP-A-0 803 533, EP-A-0 812 697, EP-A-0 824 959, EP-A-0 889080 and EP-A-1 149 624.

The main disadvantages of wet phase inversion are the limited productionspeed obtainable and the high amounts of organic solvents required.

Dry phase inversion processes are described e.g. in EP-A-1 176 030. Analternative method is disclosed in U.S. Pat. No. 4,466,931, EP-A-0 216622 and EP-A-0 481 517 describing a membrane which is produced byirradiating curable monomers in non-volatile organic solvents which areto be removed by washing with a washing liquid of low boiling point.

As many curable compounds are hydrophobic in nature both dry and wetphase inversion techniques require organic apolar solvents to obtain aclear solution. Since these membranes are often not hydrophilic anadditional process step may be required to make the membrane hydrophilicfor instance by impregnating the membrane with a saline solution as isdescribed in e.g. FR-A-2 687 589, with molecules that comprisehydrophilic and hydrophobic groups as is described in e.g. WO2004/022201or with a solution of a cationic or anionic polymer as is described ine.g. JP-A-2 107 649.

WO-A-00/63023 describes the impregnation of membranes, made by phaseinversion or by stretching, with a solution of a cationic polymer toprevent pigment ink migration.

Membranes can also be made by other methods such as thermalpolymerization as described in e.g. EP-A-0 251 511, JP-A-5 177 120 andU.S. Pat. No. 4,942,204 or grafting acrylic acid to PVC films asdescribed in GB-A-1 549 352 but these membranes or films are not porousand are not formed by phase separation from a solvent.

Membranes made from amphiphilic copolymers as free standing films aredescribed in WO-A-01/88025 which are also not porous and of relativelysmall size (up to 1 mm²).

Although under certain conditions acceptable results can be obtainedwith the above-mentioned prior art materials, there is still a need forimprovement. The present invention seeks to fulfill, at least in part,this need.

For recording media improvement is required in particular with respectto smearing properties, which may be associated with absorptionproperties of the porous film, in particular absorption speed. At thesame time a porous film must be provided that has a good gloss.

There is a need for a membrane that can be produced at high speedswithout requiring costly measures to guarantee safety and to preventpollution of the environment. This invention aims at solving theseproblems, at least in part.

DETAILED DESCRIPTION

It is an object of this invention to provide a porous membrane that canbe produced at low cost and at high coating speeds. It is a furtherobject of this invention to provide a recording medium having excellentdrying characteristics and also high image print densities and a goodwhiteness. We unexpectedly found that these objectives can be met byproviding a curable composition that comprises prior to curing at leastone type of monomer and an aqueous solvent. Preferably this monomer hasan amphiphilic character. By coating said composition on a substrate,curing the coated composition thereby causing phase separation betweenthe crosslinked compounds and the solvent a substrate provided with aporous layer is formed. Subsequently the porous membrane is impregnatedwith a solution comprising at least one cationic compound e.g. a metalion, a mordant or optical brightener thereby improving the functionalityof the membrane. The membrane may be subjected to a washing and/ordrying step. When coating such a mixture comprising a curable compoundon a substrate, followed by the subsequent steps of curing the mixture,washing and/or drying the resulting porous layer, impregnating theporous layer with a functional compound and optionally separating theporous layer from the substrate, a porous membrane can be obtained whichcan be used in various applications and which is characterized by itshigh solvent flux and/or uptake capability. If separated the porousmembrane of the present invention can be fixed afterwards to all kindsof supports. Separation from the substrate can be easily achieved byproper treatment of the substrate e.g. by applying a ‘release’ layercomprising for instance a siloxane based polymer before coating thecurable compound mixture on the substrate. The isolated porous membraneof this invention can be separately attached to a substrate via anadhesive layer. This adhesive layer can also impart certain propertiedto the resulting medium. Throughout the present text the terms curablecompound and (curable) monomer are used interchangeably. The porousmembrane in accordance with the present invention can be used in avariety of applications, such as processes for the separation,concentration and purification of gasses, liquids and mixtures.

By impregnation all kinds of additives may be brought into the porousmembrane. Preferably these additives are water soluble or may bedispersed or added as an emulsion. To maintain the porous character thetotal quantity of additives added should be lower than the total porevolume of the membrane, in other words the pores should not becompletely filled with additives. The pH of the impregnation solutionpreferably is comparable to the pH of the porous membrane or ifnecessary may be adjusted to obtain a clear solution. The impregnationsolution may be applied in a wide range of concentrations depending onthe type of additives. A suitable concentration is between 1 and 20 wt.%, between 5 and 15 wt. % is more preferred. The impregnation coatingmay be a single layer, but may also be a multilayer. A multilayer isvery suitable to direct one or more compounds to a desired region in themembrane. Compounds such as mordants and optical brighteners arepreferably present in the top region of the membrane; by impregnatingthe membrane by a multilayer wherein these compounds are present in thetop layer these compounds will be located near the surface of themembrane. The top layer of the impregnation solution is preferably anaqueous solution and may comprise mordants, optical brighteners,surfactants, curable monomers, amine synergists, water soluble polymers,transportability improving/friction reducing agents, UV-absorbers, dyefading prevention agents (radical scavengers, light stabilizers,anti-oxidants), cross-linking agents and conventional additives such aspH regulators, viscosity regulators, biocides, organic solvents.

Cationic mordants may coagulate with negatively charged curablecompounds if added to the curable composition. Therefore it is generallydesirable not to add the mordants to the composition but to apply themordants to the porous membrane after curing. This may be done byimpregnation after partial drying or after complete drying. Impregnationcan be performed e.g. by coating or by spraying a solution onto themembrane or by dipping the membrane into a solution. Metering coatingsuch as slide or slot coating is preferred. After drying a porousmembrane remains wherein the mordant molecules are trapped at the sitewhere the negative charges are build in into the matrix.

In a preferred embodiment (part of the) cationic mordants are notintroduced after curing but are combined with anionic curable compoundsin the curable composition. These anionic and cationic compounds formcomplexes in solution, which surprisingly do not precipitate but remainin solution. Those complexes appear to have a better solubility inmonomer mixtures that have a limited compatibility with water. Thislimited compatibility with water makes these monomer mixtures verysuitable to initiate phase separation. The single ionic compounds arethought to be more hydrophilic due to their charge than the complexes inwhich the charges are shielded. Also a combination of both methods(introduction in the curable composition and by impregnation) can beused.

In another embodiment substrate and porous layer are not separated togive an isolated porous membrane, but are used as formed e.g. a membranecoated on a nonwoven support or on a glossy support in which the porousmembrane can function as a colorant receiving layer when used inrecording media. This can be for example an inkjet recording medium inwhich case the colorant is an ink-solution.

In another embodiment a substrate is coated with two or more layers of acurable compound mixture. By this method porous membranes can bedesigned and prepared with varying properties throughout the porousmembrane. So an outer layer can be designed and prepared having colorantfixing properties, e.g. by introducing mordants in the outer layer, andan inner porous layer can be constructed having an optimized wateruptake capability. Alternatively to introduce the so-called backviewoption in backlit material, the outer layer is optimized for scratchresistance and the colorant fixing property is located in the layerclosest to a transparent support. Or for separation membraneapplications the porosity of the outer layer is controlled to determinethe separation characteristics while the inner layer(s) are optimized togive both strength to the membrane and allow high solvent fluxes.

In general the dry thickness of the porous membrane of this invention inisolated form may typically be between 10 μm and 500 μm, more preferablybetween 30 and 300 μm. When adhered to a substrate the membrane need notgive internal strength and the optimal thickness is based on propertiessuch as solvent uptake capacity. In the latter case the dry thickness istypically between 5 and 50 μm. When the substrate is impermeable toaqueous solvents the dry thickness is preferably between 20 and 50 μm,while when the substrate is able to absorb part of the solvent as is thecase for e.g. (coated) base paper the preferred dry thickness is between5 and 30 μm. When the porous layer is a multilayer the thickness of thevarious layers can be selected freely depending on the properties onelikes to achieve.

Many curable compounds are hydrophobic in nature and require highconcentrations of organic apolar solvents to obtain a clear solution.Large amounts of volatile organic solvents are not preferred since thesemay result in hazardous conditions in the production area during thedrying phase of the membrane while non-volatile solvents are difficultto remove and are thus not preferred either. For reasons of safety,health and the environment, as well as from economic viewpoint, water isthe most preferred solvent. It was found that suitable curable compoundsare water reducible to form an aqueous solution. A compound is regardedas water reducible when at 25° C. at least 2 wt % of water is compatiblewith the curable compound. Preferably at least 4 wt %, more preferablyat least 10 wt % of water is miscible with the curable compounds of theinvention. Preferably environmental friendly solvents, such as water,are used. A solvent comprising water is generally referred to as anaqueous solvent. The aqueous solvent preferably comprises at least 30wt. % water, more preferably at least 50 wt %, and may further compriseother polar or apolar co-solvents. In case the miscibility with water isnot sufficient to dissolve the curable compound completely admixing of aco-solvent is desirable. Preferably the solvent contains at least 60 wt.%, preferably at least 70 wt. % and more preferably at least 80 or even90 wt. % of water. In a specific embodiment the solvent is water anddoes not contain organic co-solvents. For example, 10% CN132, 27.5%CN435 and 62.5% water, or 21.5% CN132, 21.5% CN435 and 57% water, or 60%CN132 and 40% water, or 49.75% CN132, 49.75% water and 0.5%dodecyltrimethylammonium chloride can give a favorable porous matrix.CN132 and CN435 are curable monomers available from Cray Valley, France.CN132 is a low viscosity aliphatic epoxy acrylate. CN435 (available inthe US as SR9035) is an ethoxylated trimethylolpropane triacrylate.

As co-solvents, polar volatile solvents that can be sufficiently removedby drying are preferred. Preferred co-solvents are lower alkyl alcohols,alkanones, alkanals, esters, or alkoxy-alkanes. The term “lower alkyl”means that the alkyl chain contains less than 7, preferably less than 6and more preferably less than 5 carbon-atoms, preferably 1-4 carbonatoms. In one embodiment the solvent is a mixture of isopropanol andwater. Other preferred co-solvents are e.g. methanol, ethanol,1-propanol, acetone, ethyl acetate, dioxane, methoxy ethanol anddimethylformamide. Most preferred are co-solvents having a boiling pointlower than that of water.

The solubility of the curable compound in the solvent is anotherparameter of importance. Preferably the curable composition is a clearsolution. The solvent can be chosen such that the selected curablecompound or compound mixture is completely dissolved. A clear solutionis more stable and is generally preferred. However a slight turbidityusually does not cause instability and is in most cases acceptable. Onthe other hand for phase separation to occur the growing polymer shouldbe insoluble in the solvent. This puts certain restrictions to thecurable compounds that can be selected in combination with a certainsolvent. Possible methods that can facilitate the selection of suitablecombinations are described in e.g. EP-A-216622 (cloud point) and U.S.Pat. No. 3,823,027 (Hansen system).

To obtain a large difference in solubility between the initial compoundsand the resulting polymer and thus a fast phase separation preferablythe molecular weight (MW) of the initial compounds is not too large,although also with high-MW polymers porous membranes can be realized bycareful selection of the solvent. Preferably the MW of the curablemonomers or oligomers is less than 10000 Dalton, more preferably lessthan 5000 Dalton. Good results are obtained with compounds having a MWof less than 1000 Dalton.

In addition to the curable compound having a water reducibility ofbetween 2 wt % and 50 wt % other types of curable monomers may bepresent in the curable composition. Curable compounds according theinvention are described for example in “Development of ultraviolet andelectron beam curable materials” (edited by Y. Tabata, CMC publishing,2003, ISBN 4882317915) and may be selected from, but are not limited toepoxy compounds, oxetane derivatives, lactone derivatives, oxazolinederivatives, cyclic siloxanes, or ethenically unsaturated compound suchas acrylates, methacrylates, polyene-polythiols, vinylethers,vinylamides, vinylamines, allyl ethers, allylesters, allylamines, maleicacid derivaties, itacoic acid derivaties, polybutadienes and styrenes.Preferably as the main component (meth)acrylates are used, such asalkyl-(meth)acrylates, polyester-(meth)acrylates,urethane-(meth)acrylates, polyether-(meth)acrylates,epoxy-(meth)acrylates, polybutadiene-(meth)acrylates,silicone-(meth)acrylates, melamine-(meth)acrylates,phosphazene-(meth)acrylates, (meth)acrylamides and combinations thereofbecause of their high reactivity. Other types of curable compounds maybe combined with the main component in order to modify certaincharacteristics of the resulting membrane. These compounds can be usedin the form of a monomer solution, monomer suspension, monomerdispersion, oligomer solution, oligomer suspension, oligomer dispersion,polymer solution, polymer suspension and polymer dispersion.

In order to prepare the porous membrane of the invention the curablecomposition and the processing conditions have to be selected with care.Upon irradiation the monomers (or oligomers or prepolymers) crosslink togradually form polymers. During this process the solubility of thegrowing polymer in the solvent decreases resulting in phase separationand by result the polymer separates from the solution. Finally thepolymer forms a network with a porous structure wherein the solventfills the pores. Upon drying the solvent is removed and a porousmembrane remains. In certain embodiments the membrane is not dried butoptionally washed and kept in a wet condition to prevent collapsing ofthe pores. To obtain an optimal structure of the porous membrane it isimportant to carefully select the concentration of the curable compoundor mixture of curable compounds. When the concentration is too low, itis assumed that upon curing no network structure is formed and when theconcentration is too high experiments indicate that a more or lesshomogenous gelled layer may be formed that yields a non-porous,transparent layer after drying. Also when the monomers are too solublein the solvent no phase separation occurs and then usually a gelstructure is formed after polymerization. A porous structure isessential for a high solvent flux. In view of this, the concentration ofthe curable compound or compounds in the solvent is preferably between10 and 80 wt. %, more preferably between 20 and 70 wt. %, mostpreferably between 30 and 60 wt %.

When the porous membrane is used as a colorant receiving medium, e.g. aninkjet recording medium, where aqueous inks are used to form images themembrane should have a hydrophilic character in order to rapidly absorbthe aqueous solvents involved. In the case that the curable compositioncontains water as main solvent the polymer formed generally must havehydrophobic character because incompatibility with the solvent isimportant for phase separation to occur. This implies that for thisapplication the membrane of the invention must have both hydrophiliccharacter and hydrophobic character. These seemingly contradictorydemands can be realized for instance by selecting a curable compoundthat has an amphiphilic structure: a part of the molecule is hydrophilicand another part has a hydrophobic character. An amphiphilic monomer mayhave both hydrophilic and hydrophobic groups or may have amphiphilicgroups (e.g. a (1,2- or 1,3-) propylene oxide chain or a (1,2-, 1,3- or1,4-) butylene oxide chain). Examples of hydrophobic groups arealiphatic or aromatic groups, alkyl chains longer than C3 and the like.An alternative approach is to include in the curable composition curablecompounds that are hydrophilic and those that are hydrophobic. Thelatter method allows the properties of the membrane to be controlled byvarying the ratio of both types of curable compounds. Hydrophilicmonomers are for example water soluble monomers and monomers havinghydrophilic groups such as hydroxy, carboxylate, sulfate, amine, amide,ammonium, ethylene oxide chain and the like. Amphiphilicity can beobtained in several ways. Amphiphilic monomers can for instance be madeby introducing a polar group (such as hydroxy, ether, carboxylate,sulfate, amine, amide, ammonium, etc.) into the structure of ahydrophobic monomer. On the other hand starting from a hydrophilicstructure an amphiphilic monomer can be made by increasing thehydrophobic character by introducing e.g. alkyl or aromatic groups.

Good results are obtained when at least one of the curable compounds hasa restricted water reducibility. Preferably water is miscible with thecurable monomer at 25° C. in a weight ratio of between 2/98 and 50/50,more preferably between 4/96 and 50/50, even more preferably between10/90 and 50/50. Many suitable curable compounds are amphiphilic innature. A suitable concentration of the monomer can be achieved byaddition of a co-solvent, a surfactant, by adjusting the pH of thecomposition or by mixing in monomers that maintain a good solubility athigher water loads. The miscibility ratios of water with the lattermonomers are typically larger than 50 wt. % at 25° C.

Another possible method of achieving the porous membrane of theinvention is applying a mixture of a monomer having a poor miscibilitywith water, typically miscibility ratios of water with monomer at 25° C.lower than 2 wt. %, with a monomer having a good miscibility, i.e. amiscibility of water in the monomer at 25° C. larger than 50 wt. %. Manydifferent types of monomers can be successfully applied in the inventionby carefully selecting combinations of two, three or more types ofmonomers and optimizing their respective concentrations and solventcomposition.

Suitable monomers exhibiting a miscibility with water at 25° C. in aweight ratio water/monomer between 2/98 and 50/50 are: poly(ethyleneglycol) diacrylate (preferably MW<500, e.g. triethylene glycoldiacrylate, tetraethylene glycol diacrylate, etc.), ethylene glycolepoxylate dimethacrylate, glycerol diglycerolate diacrylate, propyleneglycol glycerolate diacrylate, tripropylene glycol glycerolatediacrylate, oligo(propylene glycol) diacrylate, poly(propylene glycol)diacrylate, oligo(propylene glycol) glycerolate diacrylate,poly(propylene glycol) glycerolate diacrylate, oligo(butylene oxide)diacrylate, poly(butylene oxide) diacrylate, oligo(butylene oxide)glycerolate diacrylate, poly(butylene oxide) glycerolate diacrylate,ethoxylated trimethylolpropane triacrylate (ethoxylation 3-10 mol),ethoxylated bisphenol-A diacrylate (ethoxylation 3-10 mol),2-hydroxyethyl acrylate, 2-hydroxypropylacrylate, 2-hydroxy-3-phenoxypropyl acrylate, 2-(ethoxyethoxyl)ethylacrylate,N,N′-(m)ethylene-bis(acrylamide). Also suitable are commerciallyavailable compounds such as CN129 (an epoxy acrylate), CN131B (amonofunctional aliphatic epoxy acrylate), CN133 (a trifunctionalaliphatic epoxy acrylate), CN9245 (a trifunctional urethane acrylate),CN3755 (an amino diacrylate), CN371 (an amino diacrylate), all from CrayValley, France.

Suitable (hydrophilic) monomers having a good miscibility with water(weight ratio water/monomer larger than 50/50 at 25° C.) are:poly(ethylene glycol) (meth)acrylates (preferably MW>500), poly(ethyleneglycol) di(meth)acrylates (preferably MW>500), ethoxylatedtrimethylolpropane triacrylates (ethoxylation more than 10 mol),(meth)acrylic acid, (meth)acrylamide, 2-(dimethylamino)ethyl(meth)acrylate, 3-(dimethylamino)propyl (meth)acrylate,2-(diethylamino)ethyl (meth)acrylate, 2-(dimethylamino)ethyl(meth)acrylamide, 3-(dimethylamino)propyl (meth)acrylamide,2-(dimethylamino)ethyl (meth)acrylate quartenary ammonium salt (chlorideor sulfate), 2-(diethylamino)ethyl (meth)acrylate quartenary ammoniumsalt (chloride or sulfate), 2-(dimethylamino)ethyl (meth)acrylamidequartenary ammonium salt (chloride or sulfate), 3-(dimethylamino)propyl(meth)acrylamide quartenary ammonium salt (chloride or sulfate).

Suitable (hydrophobic) monomers having a poor miscibility with water(weight ratio water/monomer smaller than 2/98 at 25° C.) are: alkyl(meth)acrylates (e.g. ethyl acrylate, n-butyl acrylate, n-hexylacrylate,octylacrylate, laurylacrylate), aromatic acrylates (phenol acrylate,alkyl phenol acrylate, etc.), aliphatic diol (di)(meth)acrylates (e.g.1,4-butanediol diacrylate, 1,6-hexanediol diacrylate, Hydroxypivalicacid neopentylglycol diacrylate, neopentylglycol diacrylate,tricyclodecanedimethanol diacrylate), trimethylolpropane triacrylate,glyceryl triacrylate, pentaerythitol triacrylate, pentaerythitoltetraacrylate, dipentaerythritol pentaacrylate, dipentaerythritolhexaacrylate, ditrimethylolpropane tetraacrylate, styrene derivatives,divinylbenzene, vinyl acetate, vinyl alkyl ethers, alkene, butadiene,norbonene, isoprene, polyester acrylates having alkyl chain longer thanC₄, polyurethane acrylates having alkyl chain longer than C₄, polyamideacrylates having alkyl chain longer than C₄.

Preferably the curable composition comprises between 1-100 wt % ofmonomers that are miscible with water in a ratio water/monomer ofbetween 2/98 and 50/50 at 25° C., more preferably between 10-80 wt %,most preferably between 40-70 wt % based on the total amount of curablemonomers. The curable composition may additionally comprise up to 99 wt%, preferably between 30-60 wt % of monomers that are miscible withwater in a ratio water/monomer larger than 50/50 at 25° C., based on thetotal amount of curable monomers. Also monomers having a poormiscibility may be present in the mixture up to 99 wt %. Another way ofobtaining a membrane according the invention is to combine between 1 and99 wt %, preferably between 30 and 80 wt % of monomers having a goodmiscibility with water and between 1-99 wt %, preferably between 10-80wt %, more preferably between 20-70 wt % of monomers that have a poormiscibility with water in a ratio water/monomer less than 2/98 at 25° C.

Photo-initiators may be used in accordance with the present inventionand can be mixed into the mixture of the curable compound(s), preferablyprior to applying the mixture to the support. Photo-initiators areusually required when the coated mixture is cured by UV or visible lightradiation. Suitable photo-initiators are those known in the art such asradical type, cation type or anion type photo-initiators.

Examples of radical type I photo-initiators are α-hydroxyalkylketones,such as 2-hydroxy-1-[4-(2-hydroxyethoxy)phenyl]-2-methyl-1-propanone(Irgacure™ 2959: Ciba), 1-hydroxy-cyclohexyl-phenylketone (Irgacure™184: Ciba), 2-hydroxy-2-methyl-1-phenyl-1-propanone (Sarcure™ SR1173:Sartomer),oligo[2-hydroxy-2-methyl-1-{4-(1-methylvinyl)phenyl}propanone] (Sarcure™SR1130: Sartomer),2-hydroxy-2-methyl-1-(4-tert-butyl-)phenylpropan-1-one,2-hydroxy-[4′-(2-hydroxypropoxy)phenyl]-2-methylpropan-1-one,1-(4-Isopropylphenyl)-2-hydroxy-2-methyl-propanone (Darcure™ 1116:Ciba); α-aminoalkylphenones such as2-benzyl-2-(dimethylamino)-4′-morpholinobutyrophenone (Irgacure™ 369:Ciba), 2-methyl-4′-(methylthio)-2-morpholinopropiophenone (Irgacure™907: Ciba); α,α-dialkoxyacetophenones such asα,α-dimethoxy-α-phenylacetophenone (Irgacure™ 651: Ciba),2,2-diethyoxy-1,2-diphenylethanone (Uvatone™ 8302: Upjohn),α,α-diethoxyacetophenone (DEAP: Rahn), α,α-di-(n-butoxy)acetophenone(Uvatone™ 8301: Upjohn); phenylglyoxolates such as methylbenzoylformate(Darocure™ MBF: Ciba); benzoin derivatives such as benzoin (Esacure™ BO:Lamberti), benzoin alkyl ethers (ethyl, isopropyl, n-butyl, iso-butyl,etc.), benzylbenzoin benzyl ethers, anisoin; mono- and bis-acylphosphineoxides, such as 2,4,6-trimethylbenzoyl-diphenylphosphine oxide (Lucirin™TPO: BASF), ethyl-2,4,6-trimethylbenzoylphenylphosphinate (Lucirin™TPO-L: BASF), bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide(Irgacure™ 819: Ciba),bis(2,6-dimethoxybenzoyl)-2,4,4-trimethyl-pentylphosphineoxide(Irgacure™ 1800 or 1870). Other commercially available photo-initiatorsare 1-[4-(phenylthio)-2-(O-benzoyloxime)]-1,2-octanedione (Irgacure™OXE01),1-[9-ethyl-6-(2-methylbenzoyl)-9H-carbazol-3-yl]-1-(O-acetyloxime)ethanone(Irgacure™ OXE02),2-hydroxy-1-{4-[4-(2-hydroxy-2-methyl-propionyl)-benzyl]-phenyl}-2-methyl-propan-1-one(Irgacure™ 127), oxy-phenyl-acetic acid 2-[2oxo-2-phenyl-acetoxy-ethoxy]-ethyl ester (Irgacure™ 754),oxy-phenyl-acetic-2-[2-hydroxy-ethoxy]-ethyl ester (Irgacure™ 754),2-(dimethylamino)-2-(4-methylbenzyl)-1-[4-(4-morpholinyl)phenyl]-1-butanone(Irgacure™ 379),1-[4-[4-benzoylphenyl)thio]phenyl]-2-methyl-2-[(4-methylphenyl)sulfonyl)]-1-propanone(Esacure™ 1001M from Lamberti),2,2′-bis(2-chlorophenyl)-4,4′,5,5′-tetraphenyl-1,2′-bisimidazole(Omnirad™ BCIM from IGM).

Examples of type II photo-initiators are benzophenone derivatives suchas benzophenone (Additol™ BP: UCB), 4-hydroxybenzophenone,3-hydroxybenzophenone, 4,4′-dihydroxybenzophenone,2,4,6-trimethylbenzophenone, 2-methylbenzophenone, 3-methylbenzophenone,4-methylbenzophenone, 2,5-dimethylbenzophenone,3,4-dimethylbenzophenone, 4-(dimethylamino)benzophenone,[4-(4-methylphenylthio)phenyl]phenyl-methanone, 3,3′-dimethyl-4-methoxybenzophenone, methyl-2-benzoylbenzoate, 4-phenylbenzophenone,4,4-bis(dimethylamino)benzophenone, 4,4-bis(diethylamino)benzophenone,4,4-bis(ethylmethylamino)benzophenone,4-benzoyl-N,N,N-trimethylbenzenemethanaminium chloride,2-hydroxy-3-(4-benzoylphenoxy)-N,N,N-trimethyl-1-propanamium chloride,4-(13-acryloyl-1,4,7,10,13-pentaoxamidecyl)benzophenone (Uvecryl™ P36:UCB),4-benzoyl-N,N-dimethyl-N-[2-(1-oxo-2-propenyl)oxy]ethylbenzenemethanaminiumchloride, 4-benzoyl-4′-methyldiphenyl sulphide, anthraquinone,ethylanthraquinone, anthraquinone-2-sulfonic acid sodium salt,dibenzosuberenone; acetophenone derivatives such as acetophenone,4′-phenoxyacetophenone, 4′-hydroxyacetophenone, 3′-hydroxyacetophenone,3′-ethoxyacetophenone; thioxanthenone derivatives such asthioxanthenone, 2-chlorothioxanthenone, 4-chlorothioxanthenone,2-isopropylthioxanthenone, 4-isopropylthioxanthenone,2,4-dimethylthioxanthenone, 2,4-diethylthioxanthenone,2-hydroxy-3-(3,4-dimethyl-9-oxo-9H-thioxanthon-2-yloxy)-N,N,N-trimethyl-1-propanaminiumchloride (Kayacure™ QTX: Nippon Kayaku); diones such as benzyl,camphorquinone, 4,4′-dimethylbenzyl, phenanthrenequinone,phenylpropanedione; dimethylanilines such as4,4′,4″-methylidyne-tris(N,N-dimethylaniline) (Omnirad™ LCV from IGM);imidazole derivatives such as2,2′-bis(2-chlorophenyl)-4,4′,5,5′-tetraphenyl-1,2′-bisimidazole;titanocenes such asbis(eta-5-2,4-cyclopentadiene-1-yl)-bis-[2,6-difluoro-3-1H-pyrrol-1-yl]phenyl]titanium(Irgacure™ 784: Ciba); Iodonium salt such as Iodonium,(4-methylphenyl)-[4-(2-methylpropyl-phenyl)-hexafluorophosphate (1-). Ifdesired combinations of photo-initiators may also be used.

For acrylates, diacrylates, triacrylates or multifunctional acrylates,type I photo-initiators are preferred. Especiallyalpha-hydroxyalkylphenones, such as 2-hydroxy-2-methyl-1-phenylpropan-1-one, 2-hydroxy-2-methyl-1-(4-tert-butyl-) phenylpropan-1-one,2-hydroxy-[4′-(2-hydroxypropoxy)phenyl]-2-methylpropan-1-one,2-hydroxy-1-[4-(2-hydroxyethoxy)phenyl]-2-methyl propan-1-one,1-hydroxycyclohexylphenylketone andoligo[2-hydroxy-2-methyl-1-{4-(1-methylvinyl)phenyl}propanone],alpha-aminoalkylphenones, alpha-sulfonylalkylphenones and acylphosphineoxides such as 2,4,6-trimethylbenzoyl-diphenylphosphine oxide,ethyl-2,4,6-trimethylbenzoylphenylphosphinate andbis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide, are preferred.Preferably the ratio of photo-initiator and curable compound(s) isbetween 0.001 and 0.1, more preferably between 0.005 and 0.05, based onweight. It is preferred to minimize the amount of photo-initiator used,in other words preferably all photo-initiator has reacted after thecuring step (or curing steps). Remaining photo-initiator may haveadverse effects such as yellowing or degradation of dyes in case themembrane is used as a recording medium. When applied as a separationmembrane excessive washing may be required to wash out remainingphoto-initiator.

When more than one layer is applied in each layer the type andconcentration of photo-initiator can be chosen independently. Forexample, in a multilayer structure the photo-initiator in the top layermay be different from the photo-initiator in lower layer(s) which cangive more efficient curing with low initiator concentrations than when asingle initiator is applied throughout all layers. Some types ofphoto-initiator are most effective in curing the surface while othertypes cure much deeper into the layer when irradiated with radiation.For the lower layers a good through cure is important and for a highefficiency of curing it is preferred to select a photo-initiator thathas an absorption spectrum not fully overlapping with the spectrum ofthe photo-initiator applied in the top layer. Preferably the differencein absorption maximum between photo-initiators in the top layer and inthe bottom layer is at least 20 nm. In the case UV radiation is used alight source can be selected having emissions at several wavelengths.The combination of UV light source and photo-initiators can be optimizedso that sufficient radiation penetrates to the lower layers to activatethe photo-initiators. A typical example is an H-bulb with an output of600 Watts/inch as supplied by Fusion UV Systems which has emissionmaxima around 220 nm, 255 nm, 300 nm, 310 nm, 365 nm, 405 nm, 435 nm,550 nm and 580 nm. Alternatives are the V-bulb and the D-bulb which havea different emission spectrum. It is obvious that there need to besufficient overlap between the spectrum of the UV light source and thatof the photo-initiators. From a choice of light sources andphoto-initiators optimal combinations can be made. The method ofmultiple types of photo-initiator allows for thicker layers to be curedefficiently with the same intensity of irradiation. Additionally byapplying different types of photo-initiator characteristics such asgloss and porosity can be optimized to levels not possible with a singletype of photo-initiator.

Curing rates may be increased by adding amine synergists to the curablecompound. Amine synergists are known to enhance reactivity and retardoxygen inhibition. Suitable amine synergists are e.g. free alkyl aminessuch as triethylamine, methyldiethanol amine, triethanol amine; aromaticamine such as 2-ethylhexyl-4-dimethylaminobenzoate,ethyl-4-dimethylaminobenzoate and also polymeric amines aspolyallylamine and its derivatives. Curable amine synergists such asethylenically unsaturated amines (e.g. (meth)acrylated amines) arepreferable since their use will give less odor, lower volatility andless yellowing due to its ability to be incorporated into the polymericmatrix by curing.

The amount of amine synergists is preferably from 0.1-10 wt % based onthe amount of curable compounds in the curable composition, morepreferably from 0.3-3 wt % based on the amount of curable compounds.

The curable compound mixture is preferably subjected to radiation toobtain the porous membrane. In principle (electromagnetic) radiation ofany suitable wavelength can be used, such as for example ultraviolet,visible or infrared radiation, as long as it matches the absorptionspectrum of the photo-initiator, when present, or as long as enoughenergy is provided to directly cure the curable compound without theneed of a photo-initiator.

Curing by infrared radiation is also known as thermal curing. Thuscuring polymerization may be effectuated by combining the ethylenicallyunsaturated monomers with a free radical initiator and heating themixture. Exemplary free radical initiators are organic peroxides such asethyl peroxide and benzyl peroxide; hydroperoxides such as methylhydroperoxide, acyloins such as benzoin; certain azo compounds such asα,α′-azobisisobutyronitrile and γ,γ′-azobis(γ-cyanovaleric acid);persulfates; peracetates such as methyl peracetate and tert-butylperacetate; peroxalates such as dimethyl peroxalate and di(tert-butyl)peroxalate; disulfides such as dimethyl thiuram disulfide and ketoneperoxides such as methyl ethyl ketone peroxide. Temperatures in therange of from about 23° C. to about 150° C. are generally employed. Moreoften, temperatures in the range of from about 37° C. to about 110° C.are used.

Irradiation by ultraviolet light is preferred. Suitable wavelengths arefor instance UV-A (400-320 nm), UV-B (320-280 nm), UV-C (280-200 nm),provided the wavelength matches with the absorbing wavelength of thephoto-initiator, if present.

Suitable sources of ultraviolet light are mercury arc lamps, carbon arclamps, low pressure mercury lamps, medium pressure mercury lamps, highpressure mercury lamps, swirlflow plasma arc lamps, metal halide lamps,xenon lamps, tungsten lamps, halogen lamps, lasers and ultraviolet lightemitting diodes. Particularly preferred are ultraviolet light emittinglamps of the medium or high pressure mercury vapor type. In addition,additives such as metal halides may be present to modify the emissionspectrum of the lamp. In most cases lamps with emission maxima between200 and 450 nm are most suitable.

The energy output of the exposing device may be between 20 and 240 W/cm,preferably between 40 and 150 W/cm but may be higher as long as thedesired exposure dose can be realized. The exposure intensity is one ofthe parameters that can be used to control the extent of curing whichinfluences the final structure of the membrane. Preferably the exposuredose is at least 40 mJ/cm², more preferably between 40 and 600 mJ/cm²,most preferably between 70 and 220 mJ/cm² as measured by an High EnergyUV Radiometer (UV Power Puck™ from EIT—Instrument Markets) in the UV-Brange indicated by the apparatus. Exposure times can be chosen freelybut need not be long and are typically less than 1 second.

In case no photo-initiator is added, the curable compound can beadvantageously cured by electron-beam exposure as is known in the art.Preferably the output is between 50 and 300 keV. Curing can also beachieved by plasma or corona exposure.

The pH of the curable compositions is preferably chosen between a valueof 2 and 11, more preferably between 3 and 8. The optimum pH depends onthe used monomers and can be determined experimentally. The curing rateappeared to be pH dependent: at high pH the curing rate is clearlyreduced resulting in a less porous membrane. At low pH values (2 andlower) yellowing of the membrane occurs upon aging which is not desiredwhen a good whiteness is preferred.

Where desired, a surfactant or combination of surfactants may be addedto the aqueous composition as a wetting agent, to adjust surfacetension, or for other purposes such as a good gloss. It is within theability of one skilled in the art to employ a proper surfactantdepending upon desired use and the substrate to be coated. Commerciallyavailable surfactants may be utilized, including radiation-curablesurfactants. Surfactants suitable for use in the curable compositioninclude nonionic surfactants, ionic surfactants, amphoteric surfactantsand combinations thereof. Preferred nonionic surfactants includeethoxylated alkylphenols, ethoxylated fatty alcohols, ethyleneoxide/propylene oxide block copolymers, fluoroalkyl ethers, and thelike. Preferred ionic surfactants include, but are not limited to, thefollowing: alkyltrimethylammonium salts wherein the alkyl group containsfrom 8 to 22 (preferably 12 to 18) carbon atoms;alkylbenzyldimethylammonium salts wherein the alkyl group contains from8 to 22 (preferably 12 to 18) carbon atoms, and ethylsulfate; andalkylpyridinium salts wherein the alkyl group contains from 8 to 22(preferably 12 to 18) carbon atoms. Surfactants may be fluorine based orsilicon based. Examples of suitable fluorosurfactants are: fluoro C₂-C₂₀alkylcarboxylic acids and salts thereof, disodiumN-perfluorooctanesulfonyl glutamate, sodium 3-(fluoro-C₆-C₁₁alkyloxy)-1-C₃-C₄ alkyl sulfonates, sodium 3-(omega-fluoro-C₆-C₈alkanoyl-N-ethylamino)-1-propane sulfonates,N-[3-(perfluorooctanesulfonamide)-propyl]-N,N-dimethyl-N-carboxymethyleneammonium betaine, perfluoro alkyl carboxylic acids (e.g. C₇-C₁₃-alkylcarboxylic acids) and salts thereof, perfluorooctane sulfonic aciddiethanolamide, Li, K and Na perfluoro C₄-C₁₂ alkyl sulfonates, Li, Kand Na N-perfluoro C₄-C₁₃ alkane sulfonyl-N-alkyl glycine,fluorosurfactants commercially available under the name Zonyl® (producedby E.I. Du Pont) that have the chemical structure ofRfCH₂CH₂SCH₂CH₂CO₂Li or RfCH₂CH₂O(CH₂CH₂O)_(x) H wherein Rf=F(CF₂CF₂)₃₋₈and x=0 to 25, N-propyl-N-(2-hydroxyethyl)perfluorooctane sulfonamide,2-sulfo-1,4-bis(fluoroalkyl)butanedioate,1,4-bis(fluoroalkyl)-2-[2-N,N,N-trialkylammonium)alkylamino]butanedioate, perfluoro C₆-C₁₀ alkylsulfonamide propyl sulfonylglycinates,bis-(N-perfluorooctylsulfonyl-N-ethanolaminoethyl)phosphonate,mono-perfluoro C₆-C₁₆ alkyl-ethyl phosphonates, andperfluoroalkylbetaine. Also useful are the fluorocarbon surfactantsdescribed e.g. in U.S. Pat. No. 4,781,985 and in U.S. Pat. No.6,084,340.

Silicon based surfactants are preferably polysiloxanes such aspolysiloxane-polyoxyalkylene copolymers. Such copolymers may be forexample dimethylsiloxane-methyl (polyoxyethylene) copolymer,dimethylsiloxane-methyl (polyoxyethylene-polyoxypropylene) siloxanecopolymer, trisiloxane alkoxylate as a copolymer of trisiloxane andpolyether, and siloxane propoxylate as a copolymer of siloxane andpolypropylene oxide. The siloxane copolymer surfactants may be preparedby any method known to those having skill in the art and can be preparedas random, alternate, block, or graft copolymers. The polyether siloxanecopolymer preferably has a weight-average molecular weight in a range of100 to 10,000. Examples of polyether siloxane copolymers commerciallyavailable in the market include SILWET DA series, such as SILWET 408,560 or 806, SILWET L series such as SILWET-7602 or COATSIL series suchas COATSIL 1211, manufactured by CK WITCO; KF351A, KF353A, KF354A,KF618, KF945A, KF352A, KF615A, KF6008, KF6001, KF6013, KF6015, KF6016,KF6017, manufactured by SHIN-ETSU; BYK-019, BYK-300, BYK-301, BYK-302,BYK-306, BYK-307, BYK-310, BYK-315, BYK-320, BYK-325, BYK-330, BYK-333,BYK-331, BYK-335, BYK-341, BYK-344, BYK-345, BYK-346, BYK-348,manufactured by BYK-CHEMIE; and GLIDE series such as GLIDE 450, FLOWseries such as FLOW 425, WET series such as WET 265, manufactured byTEGO.

Surfactants may be added in the curable composition and/or may beintroduced by impregnation of the membrane for the purpose of improvingprinter transportability, blocking resistance and waterproofness. Thesurfactant, when used, preferably is present in an amount between 0.01and 2% based on the dry weight of the membrane, more preferably between0.02 and 0.5%. Preferably the surfactants are soluble in the compositionin the concentration used. When an aqueous solvent is used preferablythe solubility of the surfactant in water at 25° C. is at least 0.5%.

For a fast uptake of especially aqueous inks the surface needs to behydrophilic. The hydrophilicity of the surface is suitably expressed bymeasuring the contact angle of water drops. Values below 80° areindicative for hydrophilic surfaces and are preferred for applicationsas ink receiving layer.

In accordance with the present invention, a membrane is referred to as“porous, nanoporous or microporous” if it contains a substantial amountof pores preferably having a diameter of between 0.0001 and 2.0 μm. Morepreferably the majority of the pores of the porous membrane of theinvention have a size of between 0.001 and 1.0 μm, even more preferablybetween 0.003 and 0.7 μm. For selected embodiments the average porediameter preferably is between 0.01 and 1.0 μm, more preferably between0.03 and 0.4 μm. There is no limitation as to the pore shape. The porescan for instance be spherical or irregular or a combination of both.Preferably the pores are inter-connected, since this will contribute toa high flux or quick solvent absorption.

The porosity of the membrane is preferably between 5 and 90 percent asdetermined by analyzing SEM cross-section images. The porosity isdetermined by the following formula:

${\frac{{Dry}\mspace{14mu}{{thickness}\mspace{14mu}\lbrack m\rbrack}}{{Coated}\mspace{14mu}{amount}\mspace{14mu}{of}\mspace{14mu}{{solids}\mspace{14mu}\left\lbrack {{kg}\text{/}m^{2}} \right\rbrack}}*100\%} - {100\%}$

wherein the density of the coated solids (matrix) is assumed to be 1kg/dm³. More preferably the porosity is between 10 and 70 percent, evenmore preferably between 20 and 50%.

For membranes applied as ink receiving layer it is important to exhibita high gloss for which the surface layer need to be smooth and the sizeand total area of the pores on the surface of the membrane must becontrolled within certain limits. A good gloss without loss in inkabsorption speed can be obtained by controlling the area occupied bypores to preferably between 0.1 and 30%. More preferably the pore areais between 0.2 and 25%, even more preferably between 0.3 and 18% formaximum gloss with high ink absorption speed. Pore area is determined bydiameter and amount of pores. This means that for a certain pore areathe amount of pores varies depending on the pore diameter. In general alow frequency of large pores is less preferred than a high frequency ofsmall pores. The absolute average pore diameter of the surface pores ispreferably smaller than 1.2 μm, more preferably between 0.02 and 1 μm,even more preferably between 0.05 and 0.7 μm. For selected embodiments arange between 0.06 and 0.3 μm is preferred. Good gloss can beadditionally expressed in a surface roughness (Ra) value. Ra values areinfluenced by pore diameter/pore area. Preferred Ra values for membraneshaving a good gloss are below 0.8 μm, more preferably below 0.5 μm, evenmore preferably below 0.3 μm and most preferably below 0.2 μm. A glossyappearance is thought to be determined mainly by the smoothness of thesurface area between the pores. In ISO 13565-1 (1998) and JIS B0671-1(2002) a method is described by which it is possible to determine the Ravalue of the surface eliminating the contribution of the pores to thecalculation. In a special embodiment the membrane is composed ofdistinct structures: an isotropic bulk matrix in the form of an openpolymer network and a thin surface layer of a completely differentstructure. This surface layer or skin layer is a continuous layer havingpores that are not connected and can be described as a perforatedcontinuous layer. By varying the process and recipe conditions thenumber and size of surface pores can be controlled according to desiredspecifications. This surface layer is thought to contribute to the glossof the membrane. For applications such as reversed osmosis it may bepreferred that there are no pores at all at the surface or only pores ofa very small diameter, which means that the skin layer can be regardedas a closed continuous layer. For the application as ink receiving layerthe surface layer is assumed to prevent the dyes present in the ink frombeing absorbed deep into the membrane which would lead to a low opticaldensity of the printed image. So the surface layer contributes to a highoptical density. On the other hand a skin layer reduces the flow ratethrough the membrane which may result in worse drying propertiesTherefore preferably this skin layer is thin, having a thickness lessthan 3 μm, more preferably the thickness of the skin layer is less than1.5 μm e.g. between 0.1 and 1.2 μm. Except for the thin skin layer themembrane is preferably symmetric, although an asymmetric structure tosome extent is allowable.

An important characteristic of the membrane is the swellability of theporous layer. In addition to the porosity the swellability contributesto the speed and capacity of solvent uptake. Depending on the desiredproperties, a certain balance can be selected between the porosity andthe swellability. To attain a certain level of solvent uptake a highporosity can be combined with a low swelling behavior or vice versa.This enables a large variation in membrane structures all with a goodsolvent uptake speed. For membranes applied as ink receiving layer theswelling preferably is between 1 and 50 μm, more preferably between 2 μmand 30 μm, most preferably between 3 and 20 μm. Because the drythickness of the porous layer may vary depending on the desiredapplication the swelling is more appropriately expressed in a relativeway as a percentage of the dry thickness. Preferably the swelling is atleast 5%, more preferably between 6 and 150% of the dry thickness of theporous membrane, even more preferably between 10 and 80%. The swellingin this invention is determined by subtracting the dry thickness of thelayer before swelling from the swollen thickness of the layer afterswelling, wherein the swollen thickness represents the thickness of thelayer after immersion in 20° C. distilled water for 3 minutes, and drythickness represents the thickness of the layer being allowed to standat 23° C. and 60% RH for more than 24 hours. The thickness of the layercan be determined by various methods. For example, there is a method inwhich after a sample is immersed in distilled water at a giventemperature for a given time to swell the layer, while the swellingprocess is observed by touching the swollen layer continuously with aneedle positioning sensor to measure the thickness of the layer beforeand after swelling. There is also a method to measure the height of theswollen layer by optical sensor without touching the surface, andsubtracting the height of the dry layer to know the swelling amount ofthe layer. The degree of swelling can be controlled by the types andratio of monomers, the extent of curing/cross-linking (exposure dose,photo-initiator type and amount) and by other ingredients (e.g. chaintransfer agents, synergists).

Surprisingly the membrane due to its swelling character showed higherimage densities when used as an ink receiving layer and an improvedozone fastness. Without wishing to be bound by theory, the researchersassume that due to swelling the colorants are incorporated in thepolymer network structure and after drying are protected against theinfluences of ozone and other gasses. In a porous network withoutswelling capability the colorants can penetrate deep into the layerwhile by swelling the colorants are thought mainly to be trapped in thesurface region of the layer explaining the increased density observed.

A disadvantage of a strongly swelling porous layer is a rather weakscratch resistance. A large swellability is achieved by a low degree ofcrosslinking which makes the structure of the membrane sensitive tophysical disturbance. Surprisingly it was found that a second curingtreatment of the dry membrane after drying is completed is moreeffective for enhancing the robustness than intensifying the curing ofthe wet coated layer. Again, without wishing to be bound by theory, theinventors suggest that by drying the unreacted curable double bonds aremoving closer to each other, thereby increasing the probability ofcrosslinking upon curing. This second curing step may be done byUV-curing, but also other methods are suitable such as EB-curing orother sources of radiation, e.g. those mentioned hereinabove. In case UVcuring is applied for the second curing at least part of thephoto-initiator need to remain in reactive form after the first curingstep. On the other hand it is important that finally essentially allphoto-initiator has reacted because remaining photo-initiator may leadto yellowing of the membrane due to aging which is undesirable forcertain applications. This can be easily achieved by tuning the initialconcentration of the photo-initiator in the recipe. Alternatively thephoto-initiator for the second curing is added separately e.g. byimpregnation.

Instead of a second curing of the membrane in the dry state the membranemay be cured while being wet. One way of execution is to perform thesecond curing shortly after the first curing without intermediate dryingstep. Another way is to prewet the dried membrane by a liquid that maycontain one or more ingredients such as surfactants. An advantage ofthis procedure is that in the wet state the membrane structure changesupon curing when the membrane is swellable in the liquid applied. Soproperties as porosity can be modified by performing a second curingstep when the membrane is in the swollen state. By this method a widerrange of materials and process conditions become suitable since tuningof the structure remains possible after the initial curing step. Inbetween both curing steps an impregnation can be carried out. Byimpregnation compounds can be brought into the membrane that are notvery well compatible with the curable composition of the first curingstep. When the structure of the membrane after the first curing isalready good, a second curing is superfluous and just drying afterimpregnation is sufficient. But when it is desired to fix the compoundsbrought in by impregnation to the matrix a second curing step is thepreferred method of crosslinking. Preferably the membrane is partlydried before an impregnation step is executed. By partial drying thecompounds introduced by impregnation e.g. by coating, spraying ordipping, can deeper penetrate into the membrane. By partial drying partof the solvent is removed, e.g. 25% or 50% and in some cases up to 80%of the solvent is removed prior to impregnation. With a good processdesign more than 2 curing steps will in general not result in improvedproperties, however certain circumstances such as limited UV intensitymay make multiple curing beneficial.

Preferably the exposure dose in the second curing step is between 80 and300 mJ/m², more preferably between 100 and 200 mJ/m² as measured by anHigh Energy UV Radiometer (UV Power Puck™ from EIT—Instrument Markets)in the UV-B range indicated by the apparatus.

The porous membrane may also comprise one or more non-curable watersoluble polymers and/or one or more hydrophilic polymers that are notcrosslinked by exposure to radiation. The non-curable water solublepolymer may be added to the curable compound mixture before curing orapplied to the cured membrane after curing.

In addition to a non-curable water soluble polymer, up to 20 wt. %crosslinking agent may be added, preferably between 0.5 and 5 wt. %,based on the amount of non-curable water soluble polymer in the layer.Suitable crosslinking agents are described in EP-A-1 437 229. Thecrosslinking agents can be used alone or in combination.

In one embodiment at least two mixtures are coated on a substrate ofwhich at least one is a curable compound mixture, which after curing anddrying results in a membrane comprising at least one top layer and atleast one bottom layer that is closer to the substrate than the toplayer. At least the top layer, and preferably also the bottom layercomprises the porous membrane of this invention. For a two-layermembrane structure the bottom layer preferably has a dry thickness ofbetween 3 and 50 μm, preferably between 7 and 40 μm, most preferablybetween 10 and 30 μm and the top layer preferably between 1 and 30 μm,preferably between 2 and 20 μm, most preferably between 4 and 15 μm.

In another embodiment a substrate is coated with at least three layersof which at least one layer, preferably the top (outer) layer comprisesa curable compound mixture. After applying the curable compositions tothe substrate, curing and drying, a membrane comprising at least threelayers is formed, which three layers then comprise at least one bottomlayer with a dry thickness of between 3 and 50 μm, preferably between 5and 40 μm, most preferably between 7 and 30 μm, at least one middlelayer with a dry thickness of between 1 and 30 μm, preferably between 2and 20 μm, most preferably between 3 and 15 μm, and at least one toplayer above the middle layer. The top layer preferably has a drythickness of less than 10 μm, preferably of between 0.1 and 8 μm, mostpreferably between 0.4 and 4 μm.

In a preferred embodiment, the substrate is coated with two, three ormore curable compound mixtures, which after curing and drying results ina recording medium in which all layers are layers comprising a porousmembrane of the invention. Said mixtures may have the same or differentcompositions depending on the results one likes to achieve. Furthermorethe curable compound mixtures might be coated simultaneously and thencured or might be coated consecutively and cured. Consecutively means,that a first mixture is coated, then cured; then a second mixture iscoated, cured and so on. In the latter situation it is likely that atleast a part of the second mixture is impregnating the first layer socare has to be taken that the pores of the resulting membrane do notbecome blocked.

Preferably the top layer and the middle layer comprising the porousmembrane of the present invention are essentially free from (porous)organic or inorganic particles that are capable of absorbing aqueoussolvents. More preferably the porous membrane is essentially free fromparticles. Essentially free means here that the amount or location ofparticles is such that there is no significant decrease in gloss orcolour density. A quantity of less than 0.1 g/m² is regarded asessentially free. Preferably all porous layers are essentially free fromparticles. An exception are matting agents, that are added to preventhandling problems such as blocking, caused by a too smooth surface andwhich preferably are added in the top layer of the medium in a lowamount. Usually less than 0.5% of the total solid content of the porouslayer(s) is formed by matting agents.

It may be desirable to add in the top layer a matting agent (also knownas anti-blocking agents) to reduce friction and to prevent imagetransfer when several printed inkjet media are stacked. Very suitablematting agents have a particle size from 1 to 20 μm, preferably between2 and 10 μm. The amount of matting agent is from 0.005 to 1 g/m²,preferably from 0.01 to 0.4 g/m². In most cases an amount of less than0.1 g/m² is sufficient. The matting agent can be defined as particles ofinorganic or organic materials capable of being dispersed in an aqueouscomposition. The inorganic matting agents include oxides such as siliconoxide, titanium oxide, magnesium oxide and aluminum oxide, alkali earthmetal salts such as barium sulphate, calcium carbonate, and magnesiumsulphate, and glass particles. The organic matting agents includestarch, cellulose esters such as cellulose acetate propionate, celluloseethers such as ethyl cellulose, and synthetic resins. The syntheticresins are water insoluble or sparingly soluble polymers which include apolymer of an alkyl(meth)acrylate, an alkoxyalkyl(meth)acrylate, aglycidyl(meth)acrylate, a (meth)acrylamide, a vinyl ester such as vinylacetate, acrylonitrile, an olefin such as ethylene, or styrene and acopolymer of the above described monomer with other monomers such asacrylic acid, methacrylic acid, alpha, beta-unsaturated dicarboxylicacid, hydroxyalkyl(meth)acrylate, sulfoalkyl(meth)acrylate and styrenesulfonic acid. Further, a benzoguanamin-formaldehyde resin, an epoxyresin, polyamide, polycarbonates, phenol resins, polyvinyl carbazol orpolyvinylidene chloride can be used. These matting agents may be usedalone or in combination.

Usually the porous membrane has an opaque appearance due to the porousstructure of the matrix. Investigations revealed that a higher imagedensity can be obtained when the outer layer or layers are somewhattransparent. This can be achieved by modifying the structure of theouter layer in such a way that the porosity is less. An additionaladvantage of a less porous top layer is a better gloss. Because solventabsorption speed is among others dependent on porosity it is preferredthat this more transparent top layer is rather thin. Because thethickness of the more transparent layer usually does not correspond withthe thickness of the top layer as coated it may be more correct to referto this layer as top region. Most effect of the transparency of the topregion on image density is obtained when the colorants are fixed in theupper layers of the membrane, preventing diffusion of the colorant tolower layers. Fixing can be achieved by incorporating into the membranemordant functionality. For instance a curable mordant can be added tothe curable composition or mordants that are non-curable can be added.Mordants are preferably added in the outer layer or layers e.g. in thetop layer and/or in the layer just below the top layer. Preferably themordants are cationic making them suitable to form complexes withanionic colorants and may be organic or inorganic. The organic andinorganic mordants may be employed alone independently or in combinationwith each other. A very suitable method to fix the mordants in the outerlayer is to introduce negative charges in the outer layer, for instanceby applying anionic curable compounds in the curable composition.

A cationic mordant described above is preferably a polymeric mordanthaving a primary to tertiary amino group or a quaternary ammonium saltas a cationic group; a cationic non-polymeric mordant may also beemployed. Such a polymeric mordant is preferably a homopolymer of amonomer (mordant monomer) having a primary to tertiary amino group or asalt thereof, or a quaternary ammonium base, as well as a copolymer or acondensation polymer of such a mordant monomer with other monomers(hereinafter referred to as a non-mordant monomers). Such a polymericmordant may be in the form either of a water-soluble polymer or awater-dispersible latex particle, e.g. a dispersion of a polyurethane.Suitable mordant monomers are for example alkyl- or benzyl ammoniumsalts comprising one or more curable groups such as vinyl, (di)allyl,(meth)acrylate, (meth)acrylamide and (meth)acryloyl groups.

A non-mordant monomer as described above is a monomer which does notcontain a basic or cationic moiety such as a primary to tertiary aminogroup or its salt, or quaternary ammonium salt and which exhibits no orsubstantially slight interaction with a dye contained in the ink jetprinting ink. Such a non-mordant monomer may for example be alkyl(meth)acrylates; cycloalkyl (meth)acrylates; aryl (meth)acrylates;aralkyl esters; aromatic vinyls; vinyl esters; allyl esters. Any of thenon-mordant monomers listed above may be employed alone or incombination with each other.

An organic mordant is preferably a polyamine or a polyallylamine and itsderivatives whose weight mean molecular weight is 100 000 or less. Apolyamine or its derivative may be any known amine polymer and itsderivative. Such a derivative may for example be a salt of a polyaminewith an acid (acid may for example be an inorganic acid such ashydrochloric acid, sulfuric acid, phosphoric acid and nitric acid, anorganic acid such as methanesulfonic acid, toluenesulfonic acid, aceticacid, propionic acid, cinnamic acid, (meth)acrylic acid and the like, acombination thereof, or those in which a part of the amine is convertedinto a salt), a derivative of a polyamine obtained by a polymericreaction, a copolymer of a polyamine with other copolymerizable monomers(such monomers may for example be (meth)acrylates, styrenes,(meth)acrylamides, acrylonitrile, vinyl esters and the like).

It is also possible to employ an inorganic mordant as a mordant,including a polyvalent water-soluble metal salt or a hydrophobic metalsalt compound. An inorganic mordant of the invention is preferably analuminum-containing compound, titanium-containing compound,zirconium-containing compound, a compound of a metal in the series ofGroup IIIB in the periodic table (salt or complex). Certain multivalentmetal ions are known to be flocculating agents; well known example arealuminum and iron(III) salts such as poly(aluminum chloride) and thesulfates of both ions. These compounds may also be applied as mordants.At high concentrations these compounds may flocculate in the presence ofother compounds in aqueous solution but at lower concentrationsapplication as a clear solution is possible.

The amount of mordant is preferably from 0.01 to 5 g/m², more preferablyfrom 0.1 to 3 g/m².

If the mordant is a relativity small molecule the mordant or themordant-colorant complex may diffuse within the layer or to other layerscausing reduced sharpness. This problem is also referred to as long termbleeding. A very good method to prevent diffusion of the mordantmolecule is to incorporate negative charges into the polymer matrix ofthe porous membrane. Preferably curable compounds bearing a negativecharge are added to the curable composition. Examples of thesenegatively charged curable compounds are ethenically unsaturatedcompounds having sulfonic or carboxylic or phosphoric acid group, ortheir metal (or ammonium) salts. Sulfonic acid derivatives are morepreferred due to stronger binding with mordants. For example,(meth)acrylic acid-(sulfoalkyl)esters such as sulfopropyl acrylic acidand sulfopropyl methacrylic acid, (meth)acryl-(sulfoalkyl)amides such as2-acryloylamido-2-methylpropane-1-sulfonic acid, styrenesulfonic acid,itaconic acid-(alkylsulfonic acid)ester, itaconicacid-bis-(alkylsulfonic acid)ester, maleicacid-(alkylsulfonicacid)ester, maleic acid-bis-(alkylsulfonicacid)ester,alkylsulfonic acid allyl ether, mercapto compounds such asmercaptoalkylsulfonic acid, and their metal/ammonium salts. When appliedthese negatively charged curable compounds are preferably added up to anamount of 30 wt. %, more preferably in an amount between 0.5 and 10 wt.% based on the weight of the curable compounds in the curablecomposition, most preferably between 1 and 5 wt. %. Better than by wt. %the introduced negative charges are expressed by equivalents since amonomer molecule may contain more than one negatively charged group andthe MW of monomers may very significantly. Preferably the porousmembrane of the invention comprises up to 10 milli equivalents (meq) perm² with a minimum of 0.1 meq/m², more preferably between 0.3 and 5meq/m², most preferably between 0.5 and 3 meq/m². The negatively chargedcompounds may be added to one composition or to the compositions formore than one layer.

Especially preferred are anionic curable compounds that comprise one ormore functional thiol groups. These compounds then act as chain transferagents which are known to be less sensitive to oxygen inhibition andhave a remarkable effect on the structure of the membrane: the porosityis less and the surface becomes smoother. Surprisingly the image densityincreases when chain transfer agents are applied, even in relatively lowamounts. An additional advantage of the use of chain transfer agents isthat the tackiness of the surface of the membrane after curing becomesless and the structure becomes more rigid. Examples includemercaptoacetic acid, mercaptopropionic acid, alkyl mercaptopropionate,mercapto-propylsulfonate, ethyldithiocarbonato-S-sulfopropylester,dimercaptopropane sulfonate and mercaptobenzimidazole sulfonate.

Alternatively chain transfer agents that are non-ionic are added inaddition to or in stead of the negatively charged curable compounds toobtain similar effects on structure and surface properties. Classes ofcompounds that comprise suitable substances are mercaptans,polymethacrylates, polyhalo alkanes, benzoquinones, oximes, anthracenes,disulfides, sulfonyl chlorides, sulfoxides, phosphines, alkyl anilines,alkyl amines and metal compounds (such as aluminum, iron, cobalt, coppersalts or complexes). Preferred compounds are mercaptoethanol,mercaptoethylether, mercaptobenzimidazole, ethyldithioacetate,butanethiol, dimethyldisulfide, tetrabromomethane, dimethylaniline,ethylenedioxydiethanethiol and triethylamine.

A special class of chain transfer agents are so-called RAFT agents(RAFT=Reversible Addition-Fragmentation chain Transfer). This RAFTreaction is a controlled radical polymerization and generally leads tovery narrow molecular weight distributions. Suitable RAFT agentscomprise a dithioester group of the formula R1-C(═S)—S—R₂, a xanthategroup of the formula R₁—O—C(═S)—S—R₂ or a thioxanthate(trithiocarbonate) group of the formula R₁—S—C(═S)—S—R₂, adithiocarbamate group of the formula R₁—NR—C(═S)—S—R₂ where R, R₁ and R₂are selected from an alkyl group, a cycloalkyl group, an aryl group, aheterocyclic group or an arenyl group. Examples are ethyldithioacetate,benzyl dithiobenzoate, cumyl dithiobenzoate, benzyl1-pyrrolecarbodithioate, cumyl 1-pyrrolecarbodithioate,o-ethyldithiocarbonato-S-(3-sulfopropyl) ester,N,N-dimethyl-S-thiobenzoylthiopropionamide,N,N-dimethyl-S-thiobenzoylthioacetamide, trithiocarbonates anddithiocarbamates.

Chain transfer agents can be characterized by a so-called chain transferconstant which preferably is larger than 0.1, more preferably largerthan 1.0. For transfer constants lower than 0.1 no or only very limitedeffects are achieved. Optimum quantities depend very much on thecomposition of the curable composition, on the type of the chaintransfer agent (reactivity) and on the irradiation dose so the optimumconcentration has to be determined case by case. At high levels of chaintransfer agents it was found that adhesion problems may occur if thecompound is in the layer adjacent to the support. When a multilayermembrane is made the chain transfer agent is preferably in the top layerwhere the effect on image density is expected to be the highest. Veryhigh levels may retard the crosslinking reaction too much resulting in adense non-porous layer or even a layer that is still uncured. Preferablythe chain transfer agent is present in an amount between 0.001 and 1.0mmol/g curable compound. For most compounds the preferred range will bebetween 0.005 and 0.1 mmol/g curable compound. If the membrane consistsof more than one layer the mentioned range apply to the layer or layerscomprising the chain transfer agent.

For adequate colorant fixing properties it is important to have asurplus of positive charges that can bind the negatively chargedcolorant molecules. Preferably the ratio of negative charges present inthe anionic curable compounds and positive charges present in thecationic compounds (e.g. mordants) is at least 1:1 and more preferablybetween 1:2 and 1:10.

In case after the membrane is made there is no surplus of positivecharges or the surplus is insufficient to fix the dyes at high printingdensities additional cationic compounds may be added in a subsequentstep, e.g. by impregnation, after the membrane has been formed. Soinitially in the curable composition the ratio of negative chargespresent in the anionic curable compounds and positive charges present inthe cationic compounds may be larger than 1, e.g. 2:1. Preferably thisratio is reduced in a subsequent step as described above by introducingmore cationic charges.

In general mordants are applied to fix the colorants (dyes) from ink.Since at least three colors are used in a colour printer and there existmany brands of ink usually a combination of mordants is required to fixall colorants. Ideally such a mix of mordants is capable of fixing allexisting dyes. Alternatively a medium is developed that is dedicated tocertain types of ink by which a higher quality may be realized than witha medium suitable for all types of ink.

Also the non-curable water soluble polymer mentioned above can bebrought into the porous membrane by impregnation.

Other additives that may be added to one or more of the curablecompositions or may be included by impregnation are UV absorbing agents,brightening agents, anti-oxidants, light stabilizing agents, radicalscavengers, anti-blurring agents, antistatic agents and/or anionic,cationic, non-ionic, and/or amphoteric surfactants.

Suitable optical brighteners are disclosed in e.g. RD11125, RD9310,RD8727, RD8407, RD36544 and Ullmann's Encyclopedia of industrialchemistry (Vol. A18 p 153-167), and comprise thiophenes, stilbenes,triazines, imidazolones, pyrazolines, triazoles, bis(benzoxazoles),coumarins and acetylenes. Preferred optical brightening agents to beused in the invention are water-soluble and comprise compounds selectedfrom the classes distyrylbenzenes, distyrylbiphenyls, divinylstilbenes,diaminostilbenes, stilbenzyl-2H-triazoles, diphenylpyrazolines,benzimidazoles and benzofurans. In a preferred embodiment the opticalbrightening agents are cationic and are trapped by negative sitespresent in the matrix. An effective method of applying these agents isby impregnation as described above. The positively charged opticalbrightening agents are preferentially trapped in the top region of theporous membrane where they have the most effect. Then lower amounts aresufficient compared with anionic agents that tend to diffuse through thecomplete layer of the membrane (or all layers in case of a multilayermembrane). Commercially available examples of suitable cationic opticalbrightening agents are Blankophor™ ACR (Bayer) and Leucophor™ FTS(Clariant).

Whiteness is suitably expressed by the b-value of the CIELAB colormodel. CIE L*a*b (CIELAB) is a color model used conventionally todescribe all the colors visible to the human eye. It was developed forthis specific purpose by the International Commission on Illumination(Commission Internationale d'Eclairage, hence the CIE acronym in itsname). The three parameters in the model represent the luminance of thecolor (L, the smallest L represents black), its position between red andgreen (a, the smallest a represents green) and its position betweenyellow and blue (b, the smallest b represents blue). For very whitemembranes low b-values are preferred, values between −5 and −8 indicatea very bright white appearance. Relatively high values (−4 and higher)indicate a more yellowish colour and are less preferred. Membranes withlower values (−8 and lower) tend to be bluish and are generally lesspreferred. The amount of optical brightening agent is preferably lowerthan 1 g/m²; more preferably between 0.004 and 0.2 g/m²; most preferablybetween 0.01 and 0.1 g/m².

Further the porous membrane may comprise one of more light stabilizingagents such as sterically hindered phenols, sterically hindered amines,and compounds as disclosed in GB2088777, RD 30805, RD 30362 and RD31980. Especially suitable are water-soluble substituted piperidiniumcompounds as disclosed in WO-A-02/55618 and compounds such as CGP-520(Ciba Specialty Chemicals, Switzerland) and Chisorb 582-L (Double BondChemical, Taiwan). Other additives may be one or more plasticizers, suchas (poly)alkylene glycol, glycerol ethers and polymer lattices with lowTg-value such as polyethylacrylate, polymethylacrylate and the like andone or more conventional additives, such as described for example inEP-A-1 437 229 and EP-A-1 419 984, and in international patentapplications WO-A-2005/032832, WO-A-2005/032834 and WO-A-2006/011800such as acids, biocides, pH controllers, preservatives, viscositymodifiers c.q. stabilizers, dispersing agents, inhibitors, anti-blurringagents, antifoam agents, anti-curling agents, water resistance-impartingagents and the like in accordance with the objects to be achieved.

The above-mentioned additives (UV absorbers, antioxidants, anti-blurringagents, plasticizers, conventional additives) may be selected from thoseknown to a person skilled in the art and may be added in a range ofpreferably from 0.01 to 10 g/m². Any of the components mentioned abovemay be employed alone or in combination with each other. They may beadded after being solubilized in water, dispersed, polymer-dispersed,emulsified, converted into oil droplets, or may be encapsulated inmicrocapsules.

When high intensity UV light is applied for cross-linking the curablecomposition heat is generated by the UV lamp(s). In many systems coolingby air is applied to prevent the lamps from becoming overheated. Still asignificant dose of IR light is irradiated together with the UV-beam. Inone embodiment the heating-up of the coated support is reduced byplacing an IR reflecting quartz plate in between the UV lamp(s) and thecoated layer that is guided underneath the lamp(s).

With this technique coating speeds up to 200 m/min or even higher, suchas 300 m/min or more, can be reached. To reach the desired dose morethan one UV lamp in sequence may be required, so that the coated layeris successively exposed to more than one lamp. When two or more lampsare applied all lamps may give an equal dose or each lamp may have anindividual setting. For instance the first lamp may give a higher dosethan the second and following lamps or the exposure intensity of thefirst lamp may be lower. Surprisingly at constant dose the relativeintensities appeared to have subtle effects on the photopolymerizationreaction, which influences the porosity and the structure. By varyingthe exposure conditions a person skilled in the art can determineoptimum settings for the process depending on the properties one wishesto achieve.

Whereas it is possible to practice the invention on a batch basis with astationary support surface, to gain full advantage of the invention, itis much preferred to practice it on a continuous basis using a movingsupport surface such as a roll-driven continuous web or belt. Using suchapparatus the curable composition can be made on a continuous basis orit can be made on a large batch basis, and the composition poured orotherwise applied continuously onto the upstream end of the drivencontinuous belt support surface, the irradiation source being locatedabove the belt downstream of the composition application station and themembrane removal station being further downstream of the belt, themembrane being removed in the form of a continuous sheet thereof.Removal of the solvent from the membrane can be accomplished eitherbefore or after the membrane is taken from the belt. For this embodimentand all others where it is desired to remove the porous membrane fromthe support surface, it is, of course, preferable that the supportsurface be such as to facilitate as much as possible the removal of themembrane therefrom. Typical of the support surfaces useful for thepractice of such embodiments are smooth, stainless steel sheet or,better yet, Teflon or Teflon-coated metal sheet. Rather than using acontinuous belt, the support surface can be of an expendable material,such as release paper or the like (but not soluble in the solvent), inthe form of a roll thereof such that it can be continuously unrolledfrom the roll, upstream of the solution application station, as acontinuous driven length and then rerolled, with the porous membranethereon, downstream of the radiation station.

It is also within the purview of the invention to form the thin layer ofsolution as a coating on or intermingled and supported by a porous sheetor fibrous web to which the resulting membrane remains bounded and whichcan function, for example, as a strengthening reinforcement or backingfor the porous membrane. Such porous support surface of which the porousmembrane is formed should, of course, be of a material which isinsoluble in the solvent used. Typical of the porous support surfaceswhich can be used for the practice of such embodiments are paper, wovenand nonwoven fabric, and the like.

Embodiments are also recognized in which the porous material is not tobe separated from a solid support, but in which the two bonded togetherare the desired final product. Examples of such embodiments arepolyester film supported porous membranes which are utilized inelectrophoretic separations, membranes attached to a transparent oropaque sheet to be used as recording media for images and the like.

As the support, any of a transparent support composed of a transparentmaterial such as a plastic, and an opaque support composed of an opaquematerial such as a paper can be used. For most membrane applications thesupport—if present—must be porous to allow the passing of fluids orgasses. These porous supports can be paper, woven and nonwoven fabric.Examples of nonwoven fabric are materials based on cellulose, polyamide,polyester, polypropylene and the like.

As a material which can be used in the transparent support for recordingmedia, materials which are transparent and have the nature of enduringthe radiated heat upon use in Overhead Projection (OHP) and back lightdisplay are preferred. Examples of these materials include polyesterssuch as polyethylene terephthalate (PET), polyethylene naphthalate(PEN), triacetate cellulose (TAC), polysulfone, polyphenylene oxide,polyethylene, polypropylene, polyvinylchloride, polyimide,polycarbonate, polyamide and the like. Other materials that may be usedas support are glass, polyacrylate and the like. Inter alia, polyestersare preferable, and polyethylene terephthalate is particularlypreferable.

The thickness of the transparent support is not particularly limited,however 50 to 200 μm is preferable from the viewpoint of the handlingproperty.

As an opaque support having high gloss, a support with the surface onwhich a colorant receiving layer is provided, having a gloss of at least5%, preferably 15% or larger, is preferable. The gloss is a valueobtained according to the method of testing the specular surface glossof the support at 750 (TAPPI T480).

Embodiments include paper supports having high gloss such as resincoated (RC) paper, baryta paper which are used in art paper, coatedpaper, cast coated paper, supports as used for silver salt photographicpaper and the like; films having high gloss by making opaque plasticfilms such as polyesters, such as polyethylene terephthalate (PET),cellulose esters such as nitrocellulose, cellulose acetate, celluloseacetate butyrate, polysulfone, polyphenylene oxide, polyimide,polycarbonate, polyamide and the like (which may have the surfacesubjected to calender treatment) by containing of a white pigment or thelike; or supports in which a covering layer of polyolefin containing ornot containing a white pigment is provided on the surface of theaforementioned various paper supports, the aforementioned transparentsupport or films containing a white pigment or the like. An example of asuitable embodiment includes a white pigment-containing expandedpolyester film (e.g. expanded PET which contains polyolefin fineparticles and in which a void is formed by stretching).

The thickness of the opaque support is not particularly limited, however50 to 300 μm is preferable from the viewpoint of the handling property.

As already mentioned an important characteristic of a recording mediumis the gloss. The gloss is preferably larger than 20% at 200, morepreferably larger than 30% as measured by a Dr. Lange Refo 3-Dreflectometer. It has been found that the gloss of the medium can beimproved by selecting the appropriate surface roughness of the usedsupport. It was found, that providing a support having a surfaceroughness characterized by the value Ra being less than 1.0 μm,preferably below 0.8 μm a very glossy medium can be obtained. A lowvalue of the Ra indicates a smooth surface. The Ra is measured accordingto DIN 4776 using a UBM equipment, software package version 1.62, withthe following settings:

(1) Point density 500 P/mm, (2) Area 5.6×4.0 mm², (3) Cut-off wavelength0.80 mm, (4) Speed 0.5 mm/sec.

In case paper is used as the support for the present invention the paperis selected from materials conventionally used in high quality printingpaper. Generally it is based on natural wood pulp and if desired, afiller such as talc, calcium carbonate, TiO₂, BaSO₄, and the like can beadded. Generally the paper also contains internal sizing agents, such asalkyl ketene dimer, higher fatty acids, paraffin wax, alkenylsuccinicacid, such as kymene, epichlorhydrin fatty acid amid and the like.Further the paper may contain wet and dry strength agents such as apolyamine, a poly-amide, polyacrylamide, poly-epichlorohydrin or starchand the like. Further additives in the paper can be fixing agents, suchas aluminum sulphate, starch, cationic polymers and the like. The Ravalue for a normal grade base paper is usually below 2.0 μm and maytypically have values between 1.0 and 1.5 μm. The porous layer of thepresent invention or layers of which at least one comprises the porouslayer of this invention can be directly applied to this base paper.

In order to obtain a base paper with a Ra value below 1.0 μm such anormal grade base paper can be coated with a pigment. Any pigment can beused. Examples of pigments are calcium-carbonate, TiO₂, BaSO₄, clay,such as kaolin, styrene-acrylic copolymer, Mg—Al-silicate, and the likeor combinations thereof. The amount being between 0.5 and 35.0 g/m² morepreferably between 2.0 and 25.0 g/m². The paper can be coated on oneside or on both sides. The amount mentioned before is the amount coatedon one side. If both sides are coated the total amount preferably isbetween 4.0 and 50 g/m². This pigmented coating can be applied as apigment slurry in water together with suitable binders likestyrene-butadiene latex, styrene-acrylate latex, methylmethacrylate-butadiene latex, polyvinyl alcohol, modified starch,polyacrylate latex or combinations thereof, by any technique known inthe art, like dip coating, roll coating, blade coating, bar coating,size press or film press. The pigment coated base paper may optionallybe calendered. The surface roughness can be influenced by the kind ofpigment used and by a combination of pigment and calendering. The basepigment coated paper substrate has preferably a surface roughnessbetween 0.4 and 0.8 μm. If the surface roughness is further reduced bysuper calendering to values below 0.4 μm the thickness and stiffnessvalues will in general become rather low.

The porous layer or layers of which at least one comprises the porouslayer of this invention, can be directly applied to the pigment coatedbase paper.

In another embodiment, the pigment coated base paper having a pigmentedtop side and a back-side is provided on at least the topside with apolymer resin through high temperature co-extrusion giving a laminatedpigment coated base paper. Typically temperatures in this (co-)extrusion method are above 280° C. but below 350° C. The preferredpolymers used are poly olefins, particularly polyethylene. In apreferred embodiment the polymer resin of the top side comprisescompounds such as an opacifying white pigment e.g. TiO₂ (anatase orrutile), ZnO or ZnS, dyes, colored pigments, including blueing agents,e.g. ultramarine or cobalt blue, adhesion promoters, opticalbrighteners, antioxidant and the like to improve the whiteness of thelaminated pigment coated base paper. By using other than white pigmentsa variety of colors of the laminated pigment coated base paper can beobtained. The total weight of the laminated pigment coated base paper ispreferably between 80 and 350 g/m². The laminated pigment coated basepaper shows a very good smoothness, which after applying the porouslayer or layers comprising the porous layer or layers of the presentinvention results in a recording medium with excellent gloss.

On the other hand, depending on the product one wants to make apolyethylene-coated paper can be used with a matt surface or silkysurface such as is well known in the art. Such a surface is obtained byconducting an embossing treatment upon extruding a polyethylene on apaper substrate.

As is evident from the description given above, the recording mediacomprising the porous layer of this invention can be a single layer or amulti-layer applied onto a support. It can also comprise layers, whichare non porous and are located below the porous layer.

The media including the inventive porous layer or layers, can be coatedin one single step or in successive steps as long as the preferred poresizes, and porosity is obtained.

As a coating method, any method can be used. For example, curtaincoating, extrusion coating, air-knife coating, slide coating, rollcoating method, reverse roll coating, dip coating, rod bar coating. Thiscoating can be done simultaneously or consecutively, depending on theembodiments used. In order to produce a sufficiently flowablecomposition for use in a high speed coating machine, it is preferredthat the viscosity does not exceed 4000 mPa·s at 25° C., more preferablythat it should not exceed 1,000 mPa·s at 25° C.

Before applying the coating to the surface of the support materialdescribed above this support may be subjected to a corona dischargetreatment, glow discharge treatment, flame treatment, ultraviolet lightirradiation treatment and the like, for the purpose of improving thewettability and the adhesiveness.

When used as recording media the membranes of the present invention canbe used for a multitude of recording applications so it is within thescope of the present invention to provide recording media that aresuitable for creating high quality images by using techniques as forexample Giclée printing, color copying, screen printing, gravure,dye-sublimation, flexography, ink jet and the like.

Except for application in (inkjet) recording media, the porous membranesfind use in a variety of other applications, such as in membranes forwater treatment, in the chemical and petrochemical industry, for ultrafiltration processes in the electrocoating of paint, in the foodindustry such as in the production process of cheese, clarification offruit juice and in the beer production, in the pharmaceutical industrywhere a high resistivity membrane for organic solvents is required, andin the biotechnology industry especially where flux reduction due tofouling by protein needs to be avoided. The membrane can be madesuitable for nanofiltration or reversed osmosis by selecting appropriateingredients and process conditions. The hydrophilic character of theporous membrane according to this invention may result in a significantreduction of the fouling rate of the membrane and makes it suitable forall kind of other application where conventional micro- and ultrafiltration is applied.

The present invention will be illustrated in more detail by thefollowing non-limiting examples. Unless stated otherwise, all givenratios and amounts are based on weight.

EXAMPLES

The following coating solutions were prepared at room temperature withconstant stirring.

Three samples were prepared. For each sample a single layer was coatedon a support using each of the three different recipes for the coatingsolutions:

TABLE 1 Composition of Single Layer Coatings Material Recipe A Recipe BRecipe C Units CN-132 133.2 133.2 133.2 gram Water 255.6 237.9 243.8gram Isopropanol 66.6 66.6 66.6 gram CN-435 88.8 88.8 88.8 gram AAMPSA5.92 gram SPI 5.92 gram GMA 5.92 gram Zonyl ™ FSN 3% 44.4 44.4 44.4 gram1 N HCl 2.8 3.3 gram 1 N NaOH 28.0 gram Irgacure ™ 2959 3.6 3.6 3.6 gram

CN-132 is an acrylate monomer supplied by Cray Valley.

CN-435 is an acrylate monomer supplied by Cray Valley.

AAMPSA is 2-acryloylamido-2-methyl-propanesulfonic acid monomer suppliedby Sigma Aldrich.

SPI is the dipotassium salt of bis-(3-sulfopropyl) ester of itaconicacid supplied by Raschig, Germany.

Irgacure™ 2959 is a photo-initiator supplied by Ciba SpecialtyChemicals.

Zonyl™ FSN is a fluoro-surfactant supplied by DuPont.

GMA is glycidylmethacrylate supplied by Sigma Aldrich.

A photographic grade paper having polyethylene laminated at both sideswas used as a support.

These solutions were coated via a slide coating machine with a coatingspeed of 24 m/min applying an amount of 40 cc/m².

1.2 seconds after coating UV curing was applied using a Light Hammer™ 6lamp of Fusion UV Systems Inc., placed in-focus, with an intensity of80%. After this process, the cured samples were dried for 3 minutes at40° C., 8% RH.

Samples:

To the three samples prepared as described above, an overcoatingsolution was applied in an amount of 15 cc/m². After drying, thewhiteness of the samples was determined using a Minolta CM1000 colourmeasurement equipment according to the CIELAB colour model. In theCIELAB model results are noted as L*a*b*, in which L* indicates theamount of reflected light, a* and b* are colour directions. For a goodwhiteness L* should be as high as possible and theoretically a* and b*should be zero to give no additional colour. But practically a somewhatmore bluish white is preferable above a pure neutral white. Theapplication of an optical brightener will reduce the b*-value resultingin a negative b*-value indicating a more bluish white. The optimumb*-value is depending on the application and on personal preference. Therange of the b*-value giving a bright white inkjet paper is from −5 to−8. Values higher than −5 are considered slightly yellowish, valueslower than −8 are bluish and are not preferred.

TABLE 2 Effect of Overcoating on Whiteness (B-Value) Sample RecipeOvercoating Recipe B-Value 1 A — −3.98 2 A 0.25 g/l ACR −4.73 3 A 1.0g/l ACR −6.65 4 A 0.25 g/l PPW −4.37 5 A 1.0 g/l PPW −4.50 6 B — −4.03 7B 0.25 g/l ACR −5.71 8 B 1.0 g/l ACR −7.11 9 B 0.25 g/l PPW −4.81 10 B1.0 g/l PPW −5.55 11 C — −3.21 12 C 0.25 g/l ACR −4.42 13 C 1.0 g/l ACR−5.90 14 C 0.25 g/l PPW −4.06 15 C 1.0 g/l PPW −5.08

ACR is Blankophor™ ACR, a cationic optical brightener from CibaSpecialty Chemicals.

PPW is Blankophor™ PPW, an anionic optical brightener from CibaSpecialty Chemicals.

For recipe A the cationic ACR is clearly better than the anionic PPW,especially at a concentration of 1.0 g/l. Recipes B and C are expectedto give better results for the cationic optical brightener becausenegative charges are incorporated in the matrix of the membrane. RecipeB gives the best results. Even at such a low concentration as 0.25 g/lACR a good whiteness is obtained while more than 1.0 g/l of PPW isrequired to reach the same level. For recipe C the absolute values arelower than for recipe A the reason of which is not yet understood.Apparently incorporating AAMPSA in the matrix structure gives betterresults than applying SPI. The relative values show an improvementversus recipe A. Compared to recipe B the differences are lessremarkable but still indicate the advantage of applying an cationicoptical brightener.

1. A process for preparing a porous membrane comprising the steps of: a.providing a mixture of at least one type of curable compound and asolvent, wherein the concentration of curable compound is between 20 and80 weight percent and wherein at least 30 weight percent of said solventis water; b. applying said mixture to a support; c. curing said curablecompound mixture by free radical polymerization, thereby causing phaseseparation between the crosslinked curable compound and the solvent; d.optionally removing said solvent by drying and/or washing the resultingporous membrane; and e. impregnating said porous membrane with animpregnating solution, wherein said solution comprises at least onecationic compound for modifying the chemical and/or physical propertiesof said porous membrane.
 2. The process according to claim 1 whereinsaid membrane is essentially free from inorganic or organic particlesthat are capable of absorbing aqueous solvent.
 3. The process accordingto claim 1 wherein the concentration of said cationic compound in saidimpregnating solution is between 1 and 20 weight percent.
 4. The processaccording to claim 1 wherein said impregnating solution comprises acompound selected from a cationic mordant, a polyether modifiedpolysiloxane derivative, a surfactant and combinations thereof.
 5. Theprocess according to claim 1 wherein the free radical polymerization iseffected by curing through irradiation by UV-light in the presence of aphoto-initiator, which photo-initiator is preferably analpha-hydroxyalkylphenone, an alpha-aminoalkylphenone, analpha-sulfonylalkylphenone, an acylphosphine oxide or a combinationthereof.
 6. The process according to claim 1 wherein said membrane isprepared by curing at least two curable compound mixtures therebyforming a membrane having at least two layers.
 7. A recording mediumcomprising a support and a porous membrane obtainable by the processaccording to any of the previous claims as receiving layer adhered tosaid support.
 8. The medium according to claim 7, wherein said receivinglayer comprises at least one cationic optical brightening agent,preferably in an amount of between 0.004 and 0.2 g/m².
 9. The mediumaccording to claim 7 wherein said membrane comprises a polyethermodified polysiloxane derivative impregnated therein.
 10. The mediumaccording to claim 7, wherein said support is a transparent supportsuitable for back-lit applications and is selected from the groupconsisting of polyethylene terephthalate (PET), polyethylene naphthalate(PEN), polysulfone, polyphenylene oxide, polyimide, polycarbonate andpolyamide.
 11. The medium according to claim 7, wherein said support isa reflective support and is selected from the group consisting of apaper support, a plastic film and a support in which a covering layer ofpolyolefin optionally containing a white pigment is provided.
 12. Amethod of printing an image comprising using Giclée printing, colourcopying, screen printing, gravure, dye-sublimation, flexography, or inkjet printing to print an image on a recording medium including a porousmembrane produced by the method of a. providing a mixture of at leastone type of curable compound and a solvent, wherein the concentration ofcurable compound is between 20 and 80 weight percent and wherein atleast 30 weight percent of said solvent is water; b. applying saidmixture to a support; c. curing said curable compound mixture by freeradical polymerization, thereby causing phase separation between thecrosslinked curable compound and the solvent; d. optionally removingsaid solvent by drying and/or washing the resulting porous membrane; ande. impregnating said porous membrane with an impregnating solution,wherein said solution comprises at least one cationic compound formodifying the chemical or physical properties of said porous membrane.